Semiconductor device and method of manufacturing semiconductor device

ABSTRACT

In a first main surface side of a silicon carbide semiconductor base, a trench is formed. A second base region of a second conductivity type is arranged at a position facing the trench in a depth direction. An end (toward a drain electrode) of the second base region of the second conductivity type, and an end (toward the drain electrode) of a first base region of the second conductivity type reach a position deeper than an end (toward the drain electrode) of a region of a first conductivity type. Thus, the electric field at a gate insulating film at the trench bottom is mitigated, suppressing the breakdown voltage of the active region and enabling breakdown voltage design of the edge termination region to be facilitated. Further, such a semiconductor device may be formed by an easy method of manufacturing.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of International ApplicationPCT/JP2016/076419 filed on Sep. 8, 2016 which claims priority from aJapanese Patent Application No. 2015-204672 filed on Oct. 16, 2015, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Embodiments of the invention relate to a semiconductor device and amethod of manufacturing a semiconductor device.

2. Description of the Related Art

Conventionally, to reduce the ON resistance of an element in a powersemiconductor device, a vertical metal oxide semiconductor field effecttransistor (MOSFET) having a trench structure is produced. In thevertical MOSFET, the trench structure in which a channel is formed to beorthogonal to a substrate surface enables the cell density per unit areato be increased more easily as compared to a planar structure in whichthe channel is formed to be parallel to the substrate surface. As aresult, the current density per unit area may be increased, which isadvantageous in terms of cost.

Nonetheless, to form the channel in an orthogonal direction when atrench structure is formed in a vertical MOSFET, a structure is adoptedin which an inner wall of the trench is entirely covered by a gateinsulating film. A portion of the gate insulating film at a bottom ofthe trench is near a drain electrode and therefore, this portion of thegate insulating film is easily subjected to high electric field. Inparticular, since ultrahigh voltage elements are produced with a widebandgap semiconductor material (semiconductor material having a widerbandgap than that of silicon such as silicon carbide (SiC)), adverseeffects on the gate insulating film at the bottom of the trenchsignificantly reduce reliability.

As a method to resolve such problems, a structure has been proposed inwhich a p-type region is formed that contacts a p-type base region andreaches a position that is deeper than that of the bottom of the trench,whereby a pn junction is formed at a position deeper than that of thebottom of the trench, to thereby mitigate the electric field strength atthe bottom of the trench (for example, refer to Japanese Patent No.5539931). Further, a structure has been proposed in which a p-typeregion is formed at the bottom of the trench (for example, refer to U.S.Pat. No. 6,180,958). A further structure has been proposed that combinesa structure in which a p-type region that contacts a p-type base regionand reaches a position that is deeper than that of the bottom of thetrench, whereby a pn junction is formed at a position deeper than thatof the bottom of the trench, and a structure in which a p-type region isformed at the bottom of the trench (for example, refer to JapaneseLaid-Open Patent Publication No. 2009-260253).

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a semiconductordevice includes a wide bandgap semiconductor substrate of a firstconductivity type containing a semiconductor material having a bandgapthat is wider than that of silicon, and a wide bandgap semiconductorlayer of the first conductivity type formed on a front surface of thewide bandgap semiconductor substrate.

The wide bandgap semiconductor layer contains a semiconductor materialhaving a bandgap that is wider than that of silicon, and an impurityconcentration of the wide bandgap semiconductor layer is lower than thatof the wide bandgap semiconductor substrate.

The semiconductor device further includes a first base region of asecond conductivity type formed selectively in a surface layer of thewide bandgap semiconductor layer of the first conductivity type, on aside of the wide bandgap semiconductor layer of the first conductivitytype opposite a wide bandgap semiconductor substrate side, and a secondbase region of the second conductivity type formed selectively in thewide bandgap semiconductor layer of the first conductivity type.

The semiconductor device further includes a region of the firstconductivity type formed selectively in the surface layer of the widebandgap semiconductor layer of the first conductivity type, on the sideof the wide bandgap semiconductor layer of the first conductivity typeopposite the wide bandgap semiconductor substrate side, an impurityconcentration of the region of the first conductivity type being higherthan that of the wide bandgap semiconductor layer of the firstconductivity type.

The semiconductor device further includes a wide bandgap semiconductorlayer of the second conductivity type formed in a surface of the widebandgap semiconductor layer of the first conductivity type on the sideof the wide bandgap semiconductor layer of the first conductivity typeopposite the wide bandgap semiconductor substrate side, the wide bandgapsemiconductor layer of the second conductivity type containing asemiconductor material having a bandgap that is wider than that ofsilicon. A source region of the first conductivity type is formedselectively in the wide bandgap semiconductor layer of the secondconductivity type.

The semiconductor device further includes a trench penetrating the widebandgap semiconductor layer of the second conductivity type and thesource region, and reaching the region of the first conductivity type, agate electrode provided in the trench via a gate insulating film, asource electrode in contact with the wide bandgap semiconductor layer ofthe second conductivity type and the source region, and a drainelectrode provided on a rear surface of the wide bandgap semiconductorsubstrate. The second base region is arranged at all positions facingthe trench in the depth direction. A part of the first base regionextends toward the trench and is in contact with the second base region.

In the embodiment, a width of the second base region is wider than awidth of the trench.

In the embodiment, the trench penetrates the region of the firstconductivity type and reaches the second base region.

In the embodiment, the region of the first conductivity type extendsbetween the wide bandgap semiconductor layer of the second conductivitytype and a connection part of a part of the first base region and thesecond base region.

In the embodiment, the semiconductor device has a planar layout in whichthe connection part of the part of the first base region and the secondbase region is arranged periodically along a direction orthogonal to adirection in which the first base region and the second base region arearranged, sandwiching the region of the first conductivity type.

In the embodiment, at least one part of an end of the first base regiontoward the drain electrode is positioned closer than an end of thesecond base region toward the drain electrode, to the drain electrode.

In the embodiment, the semiconductor device has a planar layout in whicha part of an end of the first base region toward the drain electrode,deeper than an end of the second base region toward the drain electrode,is arranged periodically along a direction orthogonal to a direction inwhich the first base region and the second base region are arranged.

In the embodiment, the semiconductor material having a bandgap widerthan that of silicon is silicon carbide.

In the embodiment, the first base region and the second base region arelaid out in a grid-like shape from a plan view.

In the embodiment, the region of the first conductivity type is providedbetween the first base region and the second base region excluding aconnection part of the first base region and the second base region.

According to another embodiment of the present invention, a method ofmanufacturing a semiconductor device includes forming a first widebandgap semiconductor layer of a first conductivity type on a frontsurface of a wide bandgap semiconductor substrate of the firstconductivity type containing a semiconductor material having a bandgapthat is wider than that of silicon, an impurity concentration of thefirst wide bandgap semiconductor layer of the first conductivity typebeing lower than that of the wide bandgap semiconductor substrate.

The method further includes selectively forming a first semiconductorregion of a second conductivity and a second semiconductor region of thesecond conductivity type in a surface layer of the first wide bandgapsemiconductor layer of the first conductivity type, and forming a secondwide bandgap semiconductor layer of the first conductivity type in asurface of the first wide bandgap semiconductor layer of the firstconductivity type. The second wide bandgap semiconductor layer of thefirst conductivity type contains a semiconductor material having abandgap that is wider than that of silicon, and an impurityconcentration of the second wide bandgap semiconductor layer of thefirst conductivity type is lower than that of the wide bandgapsemiconductor substrate.

The method further includes selectively forming a third semiconductorregion of the second conductivity type in a surface of the second widebandgap semiconductor layer of the first conductivity type, the thirdsemiconductor region of the second conductivity type being in contactwith the first semiconductor region, and forming a wide bandgapsemiconductor layer of the second conductivity type on the surface ofthe second wide bandgap semiconductor layer of the first conductivitytype, the wide bandgap semiconductor layer of the second conductivitytype containing a semiconductor material having a bandgap wider thanthat of silicon.

The method further includes selectively forming a source region of thefirst conductivity type in the wide bandgap semiconductor layer of thesecond conductivity type, and forming (e.g., all) trenches thatpenetrate the source region and the wide bandgap semiconductor layer ofthe second conductivity type and reach the second wide bandgapsemiconductor layer of the first conductivity type, at positions facingthe second semiconductor regions in a depth direction.

The method further includes forming a gate electrode in the trench via agate insulating film, forming a source electrode to be in contact withthe wide bandgap semiconductor layer of the second conductivity type andthe source region, and forming a drain electrode on a rear surface ofthe wide bandgap semiconductor substrate. In selectively forming thefirst semiconductor region of a second conductivity type and the secondsemiconductor region of the second conductivity type, a part of thefirst semiconductor region and the second semiconductor region areconnected so that the first wide bandgap semiconductor layer of thefirst conductivity type between the first semiconductor region and thesecond semiconductor region remains.

In the embodiment, the third semiconductor region is formed to be incontact with a part of the first base region other than a connectionpart with the second base region.

In the embodiment, the method includes forming a first region of thefirst conductivity type after a first process and before a thirdprocess, the first region of the first conductivity type being formedbetween the first semiconductor region and the second semiconductorregion in the surface layer of the first wide bandgap semiconductorlayer of the first conductivity type.

In the embodiment, the method includes selectively forming a secondregion of the first conductivity type after the third process and beforea fifth process, the second region of the first conductivity type formedin the surface layer of the second wide bandgap semiconductor layer ofthe first conductivity type and in contact with the first region.

In the embodiment, the method includes forming a fourth semiconductorregion of the second conductivity type after the first process andbefore the third process, the fourth semiconductor region of the secondconductivity type being formed at a position deeper than a position ofthe first semiconductor region and formed to be in contact with thefirst semiconductor region.

In the embodiment, the first semiconductor region and the secondsemiconductor region are formed to have a grid-like shape layout from aplan view.

Objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a configuration of a siliconcarbide semiconductor device according to a first embodiment;

FIG. 1B is a view of a cross-sectional structure at cutting line B-B′ inFIG. 2;

FIG. 2 is view of a planar layout at cutting line C-C in FIGS. 1A, 1B;

FIG. 3 is a cross-sectional view (part 1) of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture;

FIG. 4 is a cross-sectional view (part 2) of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture;

FIG. 5 is a cross-sectional view (part 3) of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture;

FIG. 6 is a cross-sectional view (part 4) of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture;

FIG. 7 is a cross-sectional view (part 5) of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture;

FIG. 8 is a cross-sectional view (part 6) of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture;

FIG. 9 is a cross-sectional view of an example in which positions oftrenches and second p⁺-type base regions are shifted along a horizontaldirection in an Example of the silicon carbide semiconductor deviceaccording to the first embodiment;

FIG. 10 is a characteristics diagram of critical field strength of agate insulating film of the Example of the silicon carbide semiconductordevice according to the first embodiment;

FIG. 11 is a characteristics diagram of ON resistance of the Example ofthe silicon carbide semiconductor device according to the firstembodiment;

FIG. 12 is a cross-sectional view of a configuration of the siliconcarbide semiconductor device according to a second embodiment of thepresent invention;

FIG. 13 is a cross-sectional view schematically depicting a state of thesilicon carbide semiconductor device according to the second embodimentduring manufacture; and

FIGS. 14A and 14B are current distribution diagrams for avalanchebreakdown in the comparison example and the Example of the siliconcarbide semiconductor device according to the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Manufacturing is extremely difficult when a pn junction is formed usingthe technique in Japanese Patent No. 5539931 because the breakdownvoltage cannot be secured when the pn junction is formed at a positiondeeper than that of the bottom of the trench or a position near thetrench. When a p-type region is formed using the technique in U.S. Pat.No. 6,180,958, a high electric field tends to be applied to the gateinsulating film on the side wall of the trench and the current pathnarrows in the ON state, whereby the ON resistance increases. When botha deep p-type region at a position separated from the trench and ap-type region at the bottom of the trench are formed using the techniquein Japanese Laid-Open Patent Publication No. 2009-260253, the width ofthe p-type region at the bottom of the trench is made smaller than thewidth of the trench to reduce the ON resistance and as a result, thehigh electric field applied at corner portions of the bottom of thetrench is not mitigated. Further, in Japanese Laid-Open PatentPublication No. 2009-260253, the pn junction is formed widely in aregion directly beneath the trench (trench side), whereby the breakdownvoltage of the active region rises close to the performance limitationof the wide bandgap semiconductor material. As a result, the breakdownvoltage of the active region tends to become the breakdown voltage ofthe edge termination region or higher and may reduce the capability ofthe element.

Embodiments of a semiconductor device and a method of manufacturing asemiconductor device according to the present invention will bedescribed in detail with reference to the accompanying drawings. In thepresent description and accompanying drawings, layers and regionsprefixed with n or p mean that majority carriers are electrons or holes.Additionally, + or − appended to n or p means that the impurityconcentration is higher or lower, respectively, than layers and regionswithout + or −, and represents one example. Cases where symbols such asn's and p's that include + or − are the same indicate thatconcentrations are close and therefore, the concentrations are notnecessarily equal. In the description of the embodiments below and theaccompanying drawings, main portions that are similar will be given thesame reference numerals and will not be repeatedly described. Further,in the present description, when Miller indices are described, “−” meansa bar added to an index immediately after the “−”, and a negative indexis expressed by prefixing “−” to the index.

The semiconductor device according to the present invention isconfigured using a wide bandgap semiconductor material. In theembodiments, a silicon carbide semiconductor device produced using, forexample, silicon carbide (SiC) as a wide bandgap semiconductor will bedescribed taking a MOSFET as an example. FIG. 1A is a cross-sectionalview of a configuration of the silicon carbide semiconductor deviceaccording to the first embodiment.

As depicted in FIG. 1A, in the silicon carbide semiconductor deviceaccording to the embodiment, on a first main surface (front surface),e.g., (0001) plane (Si face) of an n⁺-type silicon carbide substrate(wide bandgap semiconductor substrate of a first conductivity type) 1,an n-type silicon carbide epitaxial layer (first wide bandgapsemiconductor layer of the first conductivity type) 2 is deposited.

The n⁺-type silicon carbide substrate 1 is, for example, a siliconcarbide single-crystal substrate doped with nitrogen (N). The n-typesilicon carbide epitaxial layer 2 has an impurity concentration that islower than that of the n⁺-type silicon carbide substrate 1 and is, forexample, a low-concentration n-type drift layer doped with nitrogen. Ina surface side of the n-type silicon carbide epitaxial layer 2, oppositethe side facing the n⁺-type silicon carbide substrate 1, an n-typehigh-concentration region (region of the first conductivity type) 5 isformed. An impurity concentration of the n-type high-concentrationregion 5 is lower than that of the n⁺-type silicon carbide substrate 1and higher than that of the n-type silicon carbide epitaxial layer 2and, for example, the n-type high-concentration region 5 is ahigh-concentration n-type drift layer doped with nitrogen. Hereinafter,the n⁺-type silicon carbide substrate 1, the n-type silicon carbideepitaxial layer 2, and a p-type base layer (wide bandgap semiconductorlayer of the second conductivity type) 6 described hereinafter arecollectively regarded as a silicon carbide semiconductor base.

As depicted in FIG. 1A, on a second main surface (rear surface, i.e., arear surface of the silicon carbide semiconductor base) of the n⁺-typesilicon carbide substrate 1, a rear electrode (drain electrode) 13 isprovided. The rear electrode 13 constitutes a drain electrode. On thesurface of the rear electrode 13, a drain electrode pad 15 is provided.

On the first main surface side (p-type base layer 6 side) of the siliconcarbide semiconductor base, a trench structure is formed. In particular,a trench 16 penetrates the p-type base layer 6 from a surface on theside (first main surface side of the silicon carbide semiconductor base)of the p-type base layer 6, opposite the side facing the n⁺-type siliconcarbide substrate 1, and reaches the n-type high-concentration region 5.A gate insulating film 9 is formed along inner walls of the trench 16and at the bottom and side walls of the trench 16; and on the gateinsulating film 9 in the trench 16, a gate electrode 10 is formed. Thegate electrode 10 is insulated from the n-type silicon carbide epitaxiallayer 2 and the p-type base layer 6 by the gate insulating film 9. Thegate electrode 10 may partially protrude from a top (aspect facingtoward a source electrode pad 14) of the trench 16 toward the sourceelectrode pad 14.

In a surface layer of the n-type silicon carbide epitaxial layer 2, onthe side (first main surface side of the silicon carbide semiconductorbase) of n-type silicon carbide epitaxial layer 2 opposite the sidefacing the n⁺-type silicon carbide substrate 1, a first p⁺-type baseregion (first base region of the second conductivity type) 3 and asecond p⁺-type base region (second base region of the secondconductivity type) 4 are selectively provided. The first p⁺-type baseregion 3 reaches a position deeper on the drain side than the bottom ofthe trench 16. A lower end (end on the drain side) of the first p⁺-typebase region 3 is positioned farther on the drain side than the bottom ofthe trench 16. A lower end of the second p⁺-type base region 4 ispositioned farther on the drain side than the bottom of the trench 16.The second p⁺-type base region 4 is formed at a position facing thebottom of the trench 16 in a depth direction z. A width of the secondp⁺-type base region 4 is wider than a width of the trench 16. The bottomof the trench 16 may reach the second p⁺-type base region 4, or may bepositioned in the n-type high-concentration region 5 that is between thep-type base layer 6 and the second p⁺-type base region 4, and the bottomneed not contact the second p⁺-type base region 4. The first p⁺-typebase region 3 and the second p⁺-type base region 4, for example, aredoped with aluminum (Al).

A part 17 of the first p⁺-type base region 3 extends on the trench 16side to be connected to the second p⁺-type base region 4. In this case,the part 17 (refer to FIG. 2) of the first p⁺-type base region 3 mayhave a planar layout in which the part 17 is alternately arranged withthe n-type high-concentration region 5 in a direction (hereinafter,second direction) y orthogonal to a direction (hereinafter, firstdirection) x in which the first p⁺-type base region 3 and the secondp⁺-type base region 4 are arranged. An example of the planar layout ofthe first and second p-type base regions 3, 4 is depicted in FIG. 2.FIG. 2 is a plan view of example of the planar layout of the siliconcarbide semiconductor device according to the first embodiment. In thisexample, FIG. 1A depicts a cross-sectional structure at cutting lineA-A′ in FIG. 2; FIG. 1B depicts a cross-sectional structure at cuttingline B-B′ in FIG. 2. FIG. 2 is a planar layout at cutting line C-C inFIGS. 1A, 1B. In FIG. 2, boundaries between the first p⁺-type baseregions 3 and the second p⁺-type base regions 4 are indicated byvertical dotted lines. The first and the second p-type base regions 3, 4are depicted to be connected by the part 17 of the first p⁺-type baseregion 3 (hatched portion).

As depicted in FIG. 2, farther on the drain side than the bottom of thetrench 16, the part 17 of the first p⁺-type base region 3, for example,extends toward the trenches 16 on both sides along the first direction xand is connected to a part of the second p⁺-type base region 4. Betweenthe parts 17 of the first p⁺-type base regions 3 adjacent along thesecond direction y, a part 5 a of the n-type high-concentration region 5on the drain side is arranged, i.e., the parts 17 (part connecting thefirst and second p-type base regions 3, 4) of the first p⁺-type baseregions 3 are periodically arranged along the second direction y,sandwiching the parts 5 a of the n-type high-concentration regions 5 onthe drain side. Further, the first p⁺-type base regions 3 and the secondp⁺-type base regions 4 have a grid-like layout in a plan view. Then-type high-concentration region 5 extends between the part 17 of thefirst p⁺-type base region 3 and the p-type base layer 6, i.e., at partsexposed at the side walls of the trench 16, parts 5 b of the n-typehigh-concentration region 5 on the source side are arranged between thep-type base layer 6 and the first and second p-type base regions 3, 4,and a part of a side face of the trench 16 is covered by an n-typeregion (FIG. 1B). As a result, holes that are generated when avalanchebreakdown occurs at a junction portion of the second p⁺-type base region4 and the n-type silicon carbide epitaxial layer 2 are efficientlymigrated to a source electrode 12, whereby the load on the gateinsulating film 9 is reduced, improving reliability.

The p-type base layer (wide bandgap semiconductor layer of the secondconductivity type) 6 is provided on a base first main surface side ofthe n-type silicon carbide epitaxial layer 2. The p-type base layer 6contacts the first p⁺-type base region 3. An impurity concentration ofthe p-type base layer 6, for example, may be lower than the impurityconcentration of the first p⁺-type base region 3. As a result, to lowerthe threshold voltage even when the concentration of the p-type baselayer 6 is lowered, decreases in the breakdown voltage due topunch-through may be avoided by suppressing the spreading of a depletionlayer of the p-type base layer 6. In the p-type base layer 6, on thebase first main surface side, an n⁺ source region (source region of thefirst conductivity) 7 and a p⁺⁺ contact region (contact region of thesecond conductivity) 8 are selectively provided. Further, the n⁺ sourceregion 7 and the p⁺ contact region 8 contact each other. The n-typehigh-concentration region 5 is provided in a region between the firstp⁺-type base region 3 of the surface layer on the base first mainsurface side of the n-type silicon carbide epitaxial layer 2 and thesecond p⁺-type base region 4, and in a region between the p-type baselayer 6 and the second p⁺-type base region 4.

In FIG. 1A, while only two trench MOS structures are depicted, more MOSgate (insulated gate constituted by a metal-oxide-semiconductor)structures of a trench structure may be further arranged in parallel.

An interlayer insulating film 11 is provided on the entire first mainsurface side of the silicon carbide semiconductor base so as to coverthe gate electrode 10 embedded in the trench. The source electrode 12contacts the n⁺ source region 7 and the p⁺⁺ contact region 8 via acontact hole opened in the interlayer insulating film 11. The sourceelectrode 12 is electrically insulated from the gate electrode 10 by theinterlayer insulating film 11. On the source electrode 12, the sourceelectrode pad 14 is provided.

A method of manufacturing the silicon carbide semiconductor deviceaccording to the present embodiment will be described. FIGS. 3, 4, 5, 6,7, and 8 are cross-sectional views of the silicon carbide semiconductordevice according to the present embodiment during manufacture.

The n⁺-type silicon carbide substrate 1 containing an n-type siliconcarbide is prepared. On the first main surface of the n⁺-type siliconcarbide substrate 1, a first n-type silicon carbide epitaxial layer(first wide bandgap semiconductor layer of the first conductivity type)2 a containing a silicon carbide is formed by epitaxial growth to athickness of, for example, about 30 μm while an n-type impurity, e.g.,nitrogen atoms, is doped. The first n-type silicon carbide epitaxiallayer 2 a forms the n-type silicon carbide epitaxial layer 2. The stateup to here is depicted in FIG. 3.

Next, on the surface of the first n-type silicon carbide epitaxial layer2 a, by photolithography, a mask (not depicted) having predeterminedopenings is formed using, for example, an oxide film. A p-type impurity,e.g., aluminum atoms, is ion-implanted using the oxide film as a mask.As a result, in parts of a surface region of the first n-type siliconcarbide epitaxial layer 2 a, for example, the second p⁺-type base region(second semiconductor region of the second conductivity) 4 and a firstp-type region (first semiconductor region of the second conductivity) 3a are formed at a depth of about 0.5 μm, for example, so that a distancebetween the adjacent first p-type region 3 a and second p⁺-type baseregion 4 is about 1.5 μm. A dose amount at the time of ion implantationfor forming the first p-type regions 3 a and the second p⁺-type baseregions 4, for example, may be set such that the impurity concentrationbecomes about 5×10¹⁸/cm³. Further, the first p-type regions 3 a and thesecond p⁺-type base regions 4 may be formed to have a grid-like planview.

Next, the mask used in the ion implantation for forming the first p-typeregions 3 a and the second p⁺-type base regions 4 is removed.Subsequently, an n-type impurity, e.g., nitrogen atoms, ision-implanted. As a result, between the first p-type regions 3 a and thesecond p⁺-type base regions 4 of the surface layer of the first n-typesilicon carbide epitaxial layer 2 a, a first n-type region (first regionof the first conductivity) 5 a at, for example, a depth of about 0.5 μmor less is formed. A dose amount at the time of ion implantation forforming the first n-type regions 5 a, for example, may be set such thatthe impurity concentration becomes about 1×10¹⁷/cm³. The state up tohere is depicted in FIG. 4.

Next, on the surface of the first n-type silicon carbide epitaxial layer2 a, while an n-type impurity, e.g., nitrogen atoms, is doped, a secondn-type silicon carbide epitaxial layer (second wide bandgapsemiconductor layer of the first conductivity type) 2 b is formed byepitaxial growth so that, for example, a thickness thereof is about 0.5μm. The second n-type silicon carbide epitaxial layer 2 b and the firstn-type silicon carbide epitaxial layer 2 a are collectively the n-typesilicon carbide epitaxial layer 2. Conditions of the epitaxial growthfor forming the second n-type silicon carbide epitaxial layer 2 b, forexample, may be set so that the impurity concentration of the secondn-type silicon carbide epitaxial layer 2 b becomes about 3×10¹⁵/cm³.

Next, on the surface of the n-type silicon carbide epitaxial layer 2, byphotolithography, a mask (not depicted) having predetermined openings isformed using, for example, an oxide film. A p-type impurity, e.g.,aluminum atoms, is ion-implanted using the oxide film as a mask, wherebyin parts of the surface layer of the n-type silicon carbide epitaxiallayer 2, a second p-type region (third semiconductor region of thesecond conductivity) 3 b at a depth of, for example, about 0.5 μm isformed so that, for example, each overlaps the top of a first p-typeregion 3 a. The second p-type regions 3 b and the first p-type regions 3a are collectively the first p⁺-type base regions 3. A dose amount atthe time of the ion implantation for forming the second p-type regions 3b, for example, may be such that the impurity concentration becomesabout 5×10¹⁸/cm³.

Next, the mask used in the ion implantation for the second p-typeregions 3 b is removed. Subsequently, an n-type impurity, e.g., nitrogenatoms, is ion-implanted. As a result, in parts of the surface layer ofthe second n-type silicon carbide epitaxial layer 2 b, a second n-typeregion (second region of the first conductivity type) 5 b at a depth of,for example, about 0.5 μm is formed so as to contact the first p-typeregions 3 a, the second p⁺-type base regions 4, and the first n-typeregions 5 a. A dose amount at the time of ion implantation for thesecond n-type regions 5 b, for example, may be set so that the impurityconcentration becomes about 1×10¹⁷/cm³. The second n-type regions 5 band the first n-type regions 5 a are collectively the n-typehigh-concentration regions 5. The state up to here is depicted in FIG.5.

Next, on the surface of the n-type silicon carbide epitaxial layer 2(i.e., surfaces of the first p⁺-type base regions 3 and the secondn-type regions 5 b), while a p-type impurity, e.g., aluminum atoms, isdoped, the p-type base layer (wide bandgap semiconductor layer of thesecond conductivity type) 6 is formed by epitaxial growth and athickness thereof is, for example, about 1.3 μm. Conditions of theepitaxial growth for forming the p-type base layers 6, for example, maybe such that the concentration becomes about 4×10¹⁷/cm³, which is lowerthan the impurity concentration of the first p⁺-type base region 3. Bythe processes up to here, the n-type silicon carbide epitaxial layer 2and the p-type base layer 6 are stacked on the n⁺-type silicon carbidesubstrate 1, forming the silicon carbide semiconductor base.

Next, on the surface of the p-type base layer 6, by photolithography, amask (not depicted) having predetermined openings is formed using, forexample, an oxide film. An n-type impurity, e.g., phosphorus (P) ision-implanted using the oxide film as a mask, whereby in parts of thesurface layer of the p-type base layer 6, the n⁺ source region (sourceregion of the first conductivity) 7 is formed. A dose amount at the timeof the ion implantation for the n⁺ source regions 7 may be set suchthat, for example, the impurity concentration becomes higher than theimpurity concentration of the first p⁺-type base region 3. Next, themask used in the ion implantation for the n source regions 7 is removed.

Subsequently, on the surface of the p-type base layer 6, byphotolithography, a mask (not depicted) having predetermined openings isformed using, for example, an oxide film and a p-type impurity, e.g.,aluminum, is ion-implanted in the surface of the p-type base layer 6,using the oxide film as a mask. As a result, in parts of a surfaceregion of the p-type base layer 6, the p⁺ contact region (contact regionof the second conductivity) 8 is formed. A dose amount at the time ofthe ion implantation for the p⁺⁺ contact regions 8, for example, may beset so that the impurity concentration becomes higher than that of thesecond p⁺-type base region 4. Next, the mask used in the ionimplantation for the p⁺⁺ contact regions 8 is removed. The sequence ofthe ion implantation for the n⁺ source regions 7 and the ionimplantation for the p⁺⁺ contact regions 8 may be interchanged. Thestate up to here is depicted in FIG. 6.

Next, heat treatment (annealing) is performed and, for example, thefirst p-type regions 3 a, the second p-type regions 3 b, the n⁺ sourceregions 7, and the p⁺⁺ contact regions 8 are activated. A temperature ofthe heat treatment, for example, may be about 1700 degrees C. A periodof the heat treatment, for example, may be about 2 minutes. Asdescribed, the ion-implanted regions may be collectively activated by asingle heat treatment session, or the heat treatment may be performedfor activation each time the ion implantation is performed.

Next, on the surface of the p-type base layer 6 (i.e., surfaces of the nsource regions 7 and the p⁺⁺ contact regions 8), by photolithography, amask (not depicted) having predetermined openings is formed using, forexample, an oxide film. By dry etching or the like using the oxide filmas a mask, the trenches 16 are formed penetrating the n⁺ source regions7 and the p-type base layers 6 and reaching the n-typehigh-concentration regions 5. The bottoms of the trenches 16 may reachthe second p⁺-type base regions 4, or may be positioned in the n-typehigh-concentration regions 5 between the p-type base layers 6 and thesecond p⁺-type base regions 4. Next, the mask used for forming thetrenches 16 is removed. The state up to here is depicted in FIG. 7.

Next, the gate insulating film 9 is formed along the surfaces of the n⁺source regions 7 and the p⁺⁺ contact regions 8, and along the bottomsand the side walls of the trenches 16. The gate insulating film 9 may beformed by thermal oxidation by heat treatment at a temperature of 1000degrees C. in an oxygen atmosphere. Further, the gate insulating film 9may be formed by a deposition method by a chemical reaction such as hightemperature oxidation (High Temperature Oxide: HTO), etc.

Next, on the gate insulating film 9, a multi-crystal silicon layer isformed while, for example, phosphorus atoms are doped. The multi-crystalsilicon layer is formed so as to be embedded in the trenches 16. Themulti-crystal silicon layer is patterned and left in the trenches 16,whereby the gate electrodes 10 are formed. A part of the gate electrodes10 may protrude from the tops of the trenches 16 (the source electrodepad 14 side) toward the source electrode pad 14.

Next, for example, phosphate glass of a thickness of about 1 μm isformed so as to cover the gate insulating film 9 and the gate electrodes10 to thereby form the interlayer insulating film 11. The interlayerinsulating film 11 and the gate insulating film 9 are patterned andselectively removed to form contact holes and thereby expose the n⁺source regions 7 and the p⁺⁺ contact regions 8. Thereafter, heattreatment (reflow) is performed, planarizing the interlayer insulatingfilm 11. The state up to here is depicted in FIG. 8.

Subsequently, in the contact holes and on the interlayer insulating film11, a conductive film that constitutes the source electrode 12 isformed. The conductive film is selectively removed to leave, forexample, the source electrode 12 only in the contact holes.

Next, on the second main surface of the n⁺-type silicon carbidesubstrate 1, the drain electrode 13, for example, constituted by a (Ni)film is formed. Thereafter, for example, heat treatment at a temperatureof about 970 degrees C. is performed, forming an ohmic junction betweenthe n⁺-type silicon carbide substrate 1 and the drain electrode 13.

Next, for example, an aluminum film is formed by, for example,sputtering, so as to cover the source electrode 12 and the interlayerinsulating film 11 and a thickness thereof is, for example, about 5 μm.Thereafter, the aluminum film is selectively removed so that the portionthat remains covers the active region of the entire element whereby thesource electrode pad 14 is formed.

Next, on the surface of the drain electrode 13, for example, titanium(Ti), nickel, and gold (Au) are sequentially stacked to form the drainelectrode pad 15, whereby the semiconductor device depicted in FIG. 1 iscompleted.

FIG. 9 is a cross-sectional view of an example in which positions of thetrenches and the second p⁺-type base regions are shifted along ahorizontal direction in an Example of the silicon carbide semiconductordevice according to the first embodiment. The horizontal direction isthe direction along which the first and second p-type base regions 3, 4are arranged. Here, a misalignment amount 101 is the distance (unit: μm)along the horizontal direction between a center of the second p⁺-typebase region 4 and a center of the trench 16; a p-type base region width102 is a width of the second p⁺-type base region 4 (unit: μm); and atrench width 103 is a width of the trench 16 (unit: μm).

FIG. 10 is a characteristics diagram of the critical field strength ofthe gate insulating film of the Example of the silicon carbidesemiconductor device according to the first embodiment. FIG. 10 depictsthe results of simulation of variation of the electric field of the gateinsulating film 9 when the center of the trench 16 shifts from thecenter of the second p⁺-type base region 4 along the horizontaldirection due to misalignment of the second p⁺-type base region 4directly beneath the trench 16. In FIG. 10, the relationship of thecritical field strength of the gate insulating film 9 with respect tomisalignment of a comparison example is depicted. In the structure ofthe comparison example (not depicted), the p-type base region width 102is set to be 1 μm and the trench width 103 is set to be 1 μm. Other thanthe p-type base region width 102, the configuration of the comparisonexample is the same as that of the Example. In the structure of theExample, the p-type base region width 102 is set to be 2 μm and thetrench width 103 is set to be 1 μm.

In FIG. 10, the vertical axis represents the critical field strength(unit: MV/cm) of the gate insulating film 9 and the horizontal axisrepresents the misalignment amount 101 of the positions of the trench 16and the second p⁺-type base region 4 along the horizontal direction.FIG. 10 depicts the results of simulation of the critical field strengthof the gate insulating film 9 for the Example and the comparisonexample; and is a characteristics diagram depicting one example of therelationship between the misalignment amount 101 and the critical fieldstrength of the gate insulating film 9 when 4000V is applied to thedrain. As depicted in FIG. 10, simulation results confirm that whenvoltage is applied to the drain side, the critical field strength forthe gate insulating film 9 improves more for the Example in which thep-type base region width 102 is wider than the trench width 103, thanfor the comparison example in which the p-type base region width 102 andthe trench width 103 are the same width.

FIG. 11 is a characteristics diagram of the ON resistance of the Exampleof the silicon carbide semiconductor device according to the firstembodiment. In FIG. 11, the ON resistance characteristics for thecomparison example are also depicted. FIG. 11 depicts the results ofverification of ON resistance characteristics for the Example and thecomparison example; and is a characteristics diagram of one example ofthe ON resistance characteristics of the comparison example and theExample of the semiconductor device according to the first embodiment.In FIG. 11, the vertical axis represents the ON resistance (unit: mΩcm²)and the horizontal axis represents the p-type base region width 102(unit: μm). As depicted in FIG. 11, verification results confirm that,for example, although the ON resistance increases when the p-type baseregion width 102 is increased, the ON resistance of the Example onlyincreases about 2% from the ON resistance of the comparison example,even when the p-type base region width 102=3 μm. In FIG. 11, a plot fora case where the p-type base region width 102=1 μm is set is for thecomparison example and plots for cases other than the case where thep-type base region width 102=1 μm is set are for the Example.

From the verification results, by setting the p-type base region width102 to be wider than the trench width 103, increases of the ONresistance may be suppressed and the electric field applied to the gateinsulating film 9 may be suppressed.

In the first embodiment, although a case where the second n-type region5 b is formed by ion implantation is shown, the second n-type siliconcarbide epitaxial layer 2 b may be formed as the second n-type region 5b. In other words, the impurity concentration of nitrogen at the time ofepitaxial growth of the second n-type silicon carbide epitaxial layer 2b may be set so as to be the impurity concentration of the second n-typeregion 5 b and the ion implantation may be omitted from the method ofmanufacture. Further, the n⁺-type silicon carbide substrate 1 and then-type silicon carbide epitaxial layer 2 may be collectively the siliconcarbide semiconductor base; and in the surface layer of the base firstmain surface side of the n-type silicon carbide epitaxial layer 2, thep-type base layer 6 may be formed by ion implantation. Further, then⁺-type silicon carbide substrate 1 alone may be the silicon carbidesemiconductor base, and in the surface layer of the first main surfaceside of the n⁺-type silicon carbide substrate 1, all of the regions(including the n-type high-concentration region 5, and the first andsecond p-type base regions 3, 4) constituting the MOS gate structure maybe formed by ion implantation.

As described, according to the first embodiment, the first p⁺-type baseregion that contacts the p-type base layer is provided, whereby the pnjunction between the first p⁺-type base region and the n-type driftlayer may be formed between adjacent trenches, at a position closer tothe drain side than the bottom of the trench. Further, in the n-typedrift layer, the second p⁺-type base region is provided so as tosurround the trench bottom or so as to be deeper than the trench bottomand face the trench in the depth direction, whereby at a position nearthe bottom of the trench, the pn junction of the second p⁺-type baseregion and the n-type drift layer may be formed. In this manner, the pnjunction between the n-type drift layer and the first and second p-typebase regions is formed, whereby the application of high electric fieldon the gate insulating film of the trench bottom may be prevented.Therefore, even in a case where a wide bandgap semiconductor material isused as the semiconductor material, high breakdown voltage becomespossible. Further, the second p⁺-type base region having a width that iswider than the trench width is provided, whereby the electric field atthe corner portions of the bottom of the trenches may be mitigated andthe breakdown voltage may be further increased.

Further, according to the first embodiment, a part of the first p⁺-typebase region extends toward the trench side and is connected to thesecond p⁺-type base region, whereby holes generated when avalanchebreakdown occurs at the junction portion of the second p⁺-type baseregion and the n-type silicon carbide epitaxial layer may be efficientlymigrated to the source electrode. Therefore, when the breakdown voltageis in a high state, the ON resistance may be lowered. Further, accordingto the first embodiment, the width of the second p⁺-type base region iswider than the width of the trench, whereby even when misalignment ofthe positions of the trench and the second p⁺-type base region occursalong the horizontal direction, the second p⁺-type base region isarranged so as to surround at least one of the corner portions of thetrench bottom. As a result, a semiconductor device may be provided, thatcompared to a conventional semiconductor device, has a higher criticalfield strength at the gate insulating film and an ON resistance that ismaintained to be about the same as in the conventional semiconductordevice. Therefore, a high-voltage semiconductor device having a low ONresistance may be manufactured by a method of epitaxial growth and ionimplantation or only ion implantation, a method easier than aconventional method.

FIG. 12 is a cross-sectional view of a configuration of the siliconcarbide semiconductor device according to a second embodiment of thepresent invention. As depicted in FIG. 12, the silicon carbidesemiconductor device according to the second embodiment is structured tohave in the n-type silicon carbide epitaxial layer 2, a third p-typeregion 3 c provided so as to contact the lower end (end on the drainside) of the first p⁺-type base region 3. The third p-type region 3 cfunctions as a base region together with the p-type base layer 6 and thefirst p⁺-type base region 3.

A thickness of the third p-type region 3 c, for example, may be about0.1 μm to 0.5 μm. A width of the third p-type region 3 c may be narrowerthan a width of the first p⁺-type base region 3, for example, narrowerby 0.1 μm or more than the first p⁺-type base region 3. Further, thethird p-type region 3 c may be provided so that a thickness thereof isconstant continuously along a direction of a side wall of the firstp⁺-type base region 3 and along a direction parallel to the surface ofthe n⁺-type silicon carbide substrate 1, or may be provided in periodicdot-like shapes from a bird's eye view from the n⁺-type silicon carbidesubstrate 1.

Other configurations of the silicon carbide semiconductor deviceaccording to the second embodiment are similar to those of the siliconcarbide semiconductor device according to the first embodiment andtherefore, redundant description is omitted.

FIG. 13 is a cross-sectional view schematically depicting a state of thesilicon carbide semiconductor device according to the second embodimentduring manufacture. As depicted in FIG. 13, after the formation of thefirst p-type regions 3 a, the second p⁺-type base regions 4, and thefirst n-type regions 5 a, the mask used for the ion implantation isremoved. Thereafter, on the surface of the first n-type silicon carbideepitaxial layer 2 a, by photolithography, a mask (not depicted) havingpredetermined openings is formed using, for example, a resist. A p-typeimpurity, e.g., aluminum atoms, is ion implanted using the resist as amask. As a result, as depicted in FIG. 13, at a lower part (end on thedrain side) of the first p-type region 3 a, for example, the thirdp-type region 3 c is formed and a thickness thereof is about 0.25 μmwhile, for example, a width thereof is about 1 μm. The third p-typeregion 3 c is formed so as to contact the first p-type region 3 a. Ionenergy when the third p-type region 3 c is formed may be set to be, forexample, 700 keV so that the dose amount, for example, becomes about1×10¹⁴/cm².

Other aspects of the method of manufacturing the silicon carbidesemiconductor device according to the second embodiment are similar tothose of the method of manufacturing the silicon carbide semiconductordevice according to the first embodiment and therefore, redundantdescription is omitted.

FIGS. 14A and 14B are current distribution diagrams for avalanchebreakdown in the comparison example and the Example of the siliconcarbide semiconductor device according to the second embodiment. InFIGS. 14A and 14B, variation of plane distribution (cross-sectionalview) of current values at the time of avalanche breakdown by astructure in which the third p-type region 3 c is formed (FIG. 14B) asthe Example and a structure in which the third p-type region 3 c is notformed as the comparison example (FIG. 14A) is evaluated. As depicted inFIG. 14A, in the comparison example, it is found that avalanchebreakdown occurs in the second p⁺-type base region 4 directly beneaththe gate electrode 10, and a majority of the current flows directlybeneath the gate electrode 10. On the other hand, as depicted in FIG.14B, in the Example, it was confirmed that avalanche breakdown occurs atthe third p-type region 3 c and the current path flows from the n⁺source region 7, through the third p-type region 3 c, to the drain side.Similar results are obtained when the thickness of the third p-typeregion 3 c is 0.1 μm or greater and the width is narrower than the firstp⁺-type base region 3 by 0.1 μm or more.

As described, according to the second embodiment, similar to the firstembodiment, even in a case where a wide bandgap semiconductor is used asa semiconductor material, an effect that high breakdown voltage becomespossible is achieved. Further, according to the second embodiment, atleast a part (third p-type region) of the lower end of the first p⁺-typebase region is made narrower than the lower end of the second p⁺-typebase region, whereby at the time of avalanche breakdown, current flowsfrom the source region, through the third p-type region, to the drainside. Therefore, the electric field at the gate insulating film at thebottom of the trench may be further mitigated.

In the present invention, while a structure in which a first mainsurface of a silicon carbide substrate containing silicon carbide is a(0001) plane and a MOS gate structure is provided on the (0001) planehas been described as an example, without limitation hereto, the type ofthe wide bandgap semiconductor (for example, gallium nitride (GaN),etc.), the plane orientation of the main surface, etc. may be variouschanged. Further, in the present invention, while in the embodiments,the first conductivity type is assumed as an n-type and the secondconductivity type is assumed as a p-type, the present invention issimilarly implemented when the first conductivity type is a p-type andthe second conductivity type is an n-type.

According to the present invention, the ON resistance may be lower whenthe breakdown voltage is high. As a result, the electric field strengthat the gate insulating film at the bottom of the trench is mitigated,enabling the breakdown voltage of the active region to be suppressed andfacilitation of the breakdown voltage design of the edge terminationregion.

The semiconductor device and the method of manufacturing a semiconductordevice according to the present invention achieve an effect in thatformation is facilitated, the electric field strength at the gateinsulating film at the bottom of the trench is mitigated, and thebreakdown voltage of the active region is suppressed to therebyfacilitate breakdown voltage design of the edge termination structure.

As described, the semiconductor device according to the presentinvention is useful for high-voltage semiconductor devices used in powerconverting equipment and power supply devices such as in variousindustrial machines.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A semiconductor device comprising: a wide bandgapsemiconductor substrate of a first conductivity type containing asemiconductor material having a bandgap that is wider than a bandgap ofsilicon; a wide bandgap semiconductor layer of the first conductivitytype formed on a front surface of the wide bandgap semiconductorsubstrate, the wide bandgap semiconductor layer containing asemiconductor material having a bandgap that is wider than the bandgapof silicon, an impurity concentration of the wide bandgap semiconductorlayer being lower than an impurity concentration of the wide bandgapsemiconductor substrate; a first base region of a second conductivitytype formed selectively in a surface layer of the wide bandgapsemiconductor layer of the first conductivity type, on a side of thewide bandgap semiconductor layer of the first conductivity type oppositea wide bandgap semiconductor substrate side; a second base region of thesecond conductivity type formed selectively in the wide bandgapsemiconductor layer of the first conductivity type; a region of thefirst conductivity type formed selectively in the surface layer of thewide bandgap semiconductor layer of the first conductivity type, on theside of the wide bandgap semiconductor layer of the first conductivitytype opposite the wide bandgap semiconductor substrate side, an impurityconcentration of the region of the first conductivity type being higherthan an impurity concentration of the wide bandgap semiconductor layerof the first conductivity type; a wide bandgap semiconductor layer ofthe second conductivity type formed in a surface of the wide bandgapsemiconductor layer of the first conductivity type on the side of thewide bandgap semiconductor layer of the first conductivity type oppositethe wide bandgap semiconductor substrate side, the wide bandgapsemiconductor layer of the second conductivity type containing asemiconductor material having a bandgap that is wider than the bandgapof silicon; a source region of the first conductivity type formedselectively in the wide bandgap semiconductor layer of the secondconductivity type; a trench penetrating the wide bandgap semiconductorlayer of the second conductivity type and the source region, andreaching the region of the first conductivity type; a gate electrodeprovided in the trench via a gate insulating film; a source electrode incontact with the wide bandgap semiconductor layer of the secondconductivity type and the source region; and a drain electrode providedon a rear surface of the wide bandgap semiconductor substrate, whereinthe second base region faces the trench in a depth direction, the secondbase region being provided for an entire bottom of the trench, and apart of the first base region extends toward the trench and is incontact with the second base region.
 2. The semiconductor deviceaccording to claim 1, wherein a width of the second base region is widerthan a width of the trench.
 3. The semiconductor device according toclaim 1, wherein the trench penetrates the region of the firstconductivity type and reaches the second base region.
 4. Thesemiconductor device according to claim 1, wherein the region of thefirst conductivity type extends between the wide bandgap semiconductorlayer of the second conductivity type and a connection part of a part ofthe first base region and the second base region.
 5. The semiconductordevice according to claim 1, wherein the semiconductor device has aplanar layout in which a connection part of a part of the first baseregion and the second base region is arranged periodically along adirection orthogonal to a direction in which the first base region andthe second base region are arranged, sandwiching the region of the firstconductivity type.
 6. The semiconductor device according to claim 1,wherein at least one part of an end of the first base region toward thedrain electrode is positioned closer than an end of the second baseregion toward the drain electrode, to the drain electrode.
 7. Thesemiconductor device according to claim 6, wherein the at least one partof the end of the first base region toward the drain electrode has athickness that is constant continuously along a direction of a side wallof the first base region and along a direction parallel to the frontsurface of the wide bandgap semiconductor substrate.
 8. Thesemiconductor device according to claim 1, wherein the semiconductormaterial having the bandgap wider than the bandgap of silicon is siliconcarbide.
 9. The semiconductor device according to claim 1, wherein thefirst base region and the second base region are laid out in a grid-likeshape from a plan view.
 10. The semiconductor device according to claim1, wherein the region of the first conductivity type is provided betweenthe first base region and the second base region excluding a connectionpart of the first base region and the second base region.